In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
There are many known types of nuclear fuel for both research and power producing nuclear reactors. The fuel can be of many types, including both fissionable (fissile) isotopes and fissile-producing (fertile) isotopes of Uranium, Plutonium, or Thorium, forming as example ceramic carbides, oxides, nitrides, or silicides. With the near complete dominance of current generation of light-water reactors (LWR's) for nuclear power production uranium dioxide (UO2) pellets have become the de facto standard nuclear fuel. The UO2 pellet is used in a pressured water reactor (PWR) and the boiling water reactor (BWR) configurations, being mass-produced through a ceramic processing route: once a powder of appropriate purity and fissile isotope enrichment is achieved it is pressed and then sintered in the presence of hydrogen and taken to final dimension by center-less grinding. Once the finished product is qualified it is placed inside a zirconium alloy tube and weld-sealed in an inert helium environment. This zirconium tube, during normal reactor operation, serves a number of functions including the plenum (barrier) for containment of the radiotoxic fission product gases.
Another type of reactor is a high-temperature Gas-Cooled Reactor (HTGR). The HTGR reactors, whether in the prismatic or pebble-bed configuration, utilize a fuel specifically engineered as a primary barrier to fission product retention. This is achieved through engineering layers of carbon, graphite and silicon carbide (SiC) around a fissile-material-bearing (U, Pu, etc.) fuel kernel. In this design, the SiC coating layer specifically becomes a pressure vessel. Such a structure is known as a tristructure isotropic (TRISO) fuel. An example of a traditional TRISO fuel is illustrated by a schematic showing the layers in FIG. 1. Specifically, the traditional TRISO 10 of FIG. 1 includes a fissile fuel kernel 11, a buffer graphitic layer 12, an inner pyrolytic carbon layer 13, silicon carbide layer 14, and an outer pyrolytic carbon layer 15. The traditional TRISO 10 can be pressed into a host graphite matrix (not shown) and used in a small number of commercial power reactors.
More recently, a fuel form has been developed whereby TRISO fuel, rather than being compacted within a graphite matrix, as is the case for HTGR, is compacted within a strong and impermeable fully dense SiC matrix. That fuel has been developed and previously described as a more robust fuel whereby the SiC layer of the microencapsulated “TRISO” fuel and the dense ceramic SiC matrix into which they are contained provide two barriers to fission product release in addition to any external cladding that may be present. A secondary barrier to fission product release significantly enhances the safety aspects of a nuclear fuel and reduces the related safety systems of modern LWR's, as well as benefiting gas-cooled reactors.